The chairman of the Senate committee that oversees mergers and antitrust issues sent a pointed letter to Comcast on Monday asking the company to strengthen its commitment to net neutrality by pledging never to create Internet fast lanes.

Senator Patrick Leahy, the Vermont Democrat who chairs the Judiciary Committee, called on Comcast to commit to never allow so-called fast lanes, or paid prioritization, on its network, even beyond the expiration of a current promise to avoid the practice through 2018.

Paid prioritization is a practice in which a content provider like Netflix pays an Internet service provider like Comcast to obtain preference for its data on the provider’s network.

Comcast is seeking to take over Time Warner Cable, and that deal was the subject of an antitrust hearing before Mr. Leahy’s committee in April. The letter comes as the Justice Department’s antitrust division and the Federal Communications Commission consider the antitrust implications of the proposed deal.
Mr. Leahy urged Comcast to take the step “regardless of whether” the merger is approved.

“I ask Comcast to pledge that it will not engage in paid prioritization,” Mr. Leahy wrote. “These types of arrangements pose a significant threat of dividing the Internet into those who can afford to compete and those who cannot.”

Comcast has made that commitment through 2018 as a condition of its earlier takeover of NBC Universal. And the company has repeatedly said that it has “no plans” to engage in paid prioritization. A spokeswoman for Comcast said the company was “reviewing the letter.”

In comments to the F.C.C. last month as part of the commission’s effort to write new net neutrality regulations, Comcast said it “doesn’t prioritize Internet traffic or have paid fast lanes and we have no plans to do so.”

In February, Comcast struck a deal with Netflix for that company to pay Comcast a fee to connect directly into its Internet network, in order to make sure that delays in the transmission of Netflix content were minimized.

After first praising the agreement, Netflix has said that the deal was de facto paid prioritization and denounced the practice. Comcast says the agreement is not paid prioritization but paid peering, a common practice in which content providers pay a data transport provider to establish a connection into the Internet backbone.